1. Field of the Invention
The present invention relates to a technology for scanning a medium using an optical scanning device.
2. Description of the Related Art
In recent years, a scanning optical system is used in image forming apparatuses. The scanning optical system exposes a photosensitive element with a beam emitted from a semiconductor laser. In an embodiment of the present invention, the configuration of an optical scanning device that does not degrade the quality of output images is explained. More specifically, the degradation is caused by individual variability of optical components that form the optical system including a plurality of scanning/exposing units, and caused by changes in their environment.
The optical scanning device forms a beam spot on a surface to be scanned by deflecting light beams from a light source by a rotating polygon mirror and focusing the deflected light beams toward the surface using a scanning and imaging optical system that includes an fθ lens, to scan the surface with the formed beam spot. The optical scanning device configured in this manner is widely known related to image forming apparatuses such as optical printers, optical plotters, and digital copiers.
The image forming apparatus using the optical scanning device employs an image writing process, as one process in an image forming process, for writing an image by optical scanning. The quality of the image formed in the image forming process depends on the quality of the optical scanning in the image writing process. The quality of the optical scanning depends on scanning characteristics in a main scanning direction and a sub-scanning direction of the optical scanning device.
One of the scanning characteristics in the main scanning direction is constant-speed scanning performance. For example, when a rotating polygon mirror is used as a light deflector, the light beams are deflected at a constant angular velocity. Therefore, to realize the constant-speed scanning performance, a scanning and imaging optical system in which an fθ characteristic is corrected is used.
However, it is not easy to precisely correct the fθ characteristic due to other performances required of the scanning and imaging optical system. Therefore, optical scanning by an actual scanning optical system is not performed at a constant speed and the constant speed performance as the scanning characteristic deviates from ideal constant speed scanning.
The scanning characteristic in the sub-scanning direction includes bending of the scanning line (hereinafter, “scanning line bending”) or a tilt of the scanning line. The scanning line is the path traversed by the beam spot on the surface to be scanned, and a straight line is ideal. Therefore, the optical scanning device is designed so that the scanning line is made to be a straight line. However, bending occurs in the scanning line caused by a machining error or an assembly error of optical elements and machine components.
When an imaging mirror is used as the scanning and imaging optical system and an angle is formed in the sub-scanning direction between an incidence direction of a deflected light beams into the imaging mirror and its reflection direction, bending occurs in a scanning line in principle. Even when the scanning and imaging optical system is configured as a lens system, the scanning line bending also inevitably occurs in a multibeam scanning system for optically scanning the surface with a plurality of beam spots that separate from one another in the sub-scanning direction.
The tilt of the scanning line is such that the scanning line is not orthogonal to the sub-scanning direction, which is one of scanning line bending. Therefore, in the following, explanation is given in such a manner that the tilt of the scanning line is included in an expression such as the scanning line bending, unless otherwise specified.
When an image is a monochrome image, which is written and formed by a single optical scanning device, and if scanning line bending and imperfect-constant speed performance (displacement from ideal constant speed scanning) are suppressed to some extent, visually recognized deformation does not occur in the formed image. Even so, it is desirable to reduce distortion of the image.
In addition to the monochrome image, there is a three-color image including magenta, cyan, and yellow, or a four-color image including black added to the three colors. The three-color image or the four-color image is formed as color-component images and these color-component images are superposed on each other to form a color image. This system of forming the color image is conventionally used in color copiers.
One of systems of forming a color image in the above manner is a tandem-type image forming system, which forms images for the respective color components on photosensitive elements for the respective color components. In this image forming system, if there is a variation in mutual scanning positions of optical scanning devices with respect to photosensitive elements or there is a difference in bending and a tilt of scanning lines, an abnormal image called color misregistration appears in the formed color image, which causes the quality of the color image to be degraded.
Japanese Patent Application Laid-Open No. 2002-258189 discloses an optical scanning device, as a unit of reducing bending or a tilt of the scanning line, which corrects the bending or the tilt of the scanning line by bending a long lens at a plurality of supports as support points or by tilting the long lens in the sub-scanning direction.
As a method of reinforcing an optical element, Japanese Patent Application Laid-Open No. H11-142767 discloses an example that a reinforcing plate made of a glass plate is adhered to the rear face of a mirror with a damping material, while Japanese Patent Application Laid-Open No. 2000-241733 discloses an example that a plate glass is bonded to the side face of a lens.
In an image forming apparatus using a Carlson process, a latent image formation, development, and transfer are sequentially performed according to a rotation of a photosensitive drum.
In a multi-color image forming apparatus, in which a plurality of photosensitive drums are arranged along a conveying direction of a transfer element and toner images formed on image forming stations for respective colors are superposed one another to create a multi-color image, color misregistration or color change occurs due to misregistration of the toner images in the sub-scanning direction and image quality is deteriorated. The misregistration occurs while forming a latent image and transferring the latent image. More specifically, the misregistration is caused by different spaces between the photosensitive drums for the respective colors due to eccentricity of or a variation in a diameter of each photosensitive drum, or caused by speed fluctuation or meandering of a transfer element such as a transfer belt or a conveyor belt for conveying recording paper.
Similarly, in the optical scanning device, if main-scanning magnifications and writing positions of electrostatic latent images formed on the respective photosensitive drums are not accurate with each other, the mismatches cause misregistration, which leads to color misregistration and color change.
Conventionally, the misregistration is not specified whether it is caused by the optical scanning device or caused by anything else other than the optical scanning device. Technologies for preventing the misregistration are disclosed in Japanese Patent Application Kokoku No. H07-19084 and Japanese Patent Applications Laid-Open Nos. 2001-253113 and 2003-154703. In the technologies, the misregistration is periodically detected using a misregistration detection pattern recorded on a transfer element upon startup of the device or between jobs, the position of a first line in the sub-scanning direction is corrected by matching write-start timing on every one facet of a polygon mirror which is a light deflector, and a write-start position in the main scanning direction is corrected by adjusting a timing from a synchronization detection signal generated at a scanning start point.
Furthermore, as described in Japanese Patent Application Laid-Open No. H09-58053, the magnifications of the full width between colors are made coincident with each other by detecting a scanning time from a scanning start point to a scanning end point, and by tuning frequencies of pixel clocks to each other.
On the other hand, in the multi-color image forming apparatus, an operation speed is increasing and a density of an image to be created is rising. As measures against this, there is a method of increasing the rotational speed of the polygon mirror. However, a bearing life is limited, so that heat generation and vibration cannot be suppressed. Therefore, a system using a multibeam light source capable of realizing increased speed and increased density is proposed. The increased speed and increased density can be realized by simultaneously scanning a plurality of beams at a lower rotational speed.
However, for the multibeam light source, there are a pitch error and a difference in wavelength between light sources. Japanese Patent Application Laid-Open No. 2003-154703 discloses a technology for avoiding displacement between the light sources by individually detecting misregistration of a plurality of lines as a set.
Furthermore, Japanese Patent Application Laid-Open Nos. 2003-233094 and 2003-215484 disclose technologies for using a liquid-crystal deflecting element as a unit of correcting a scanning position in the sub-scanning direction.
Moreover, Japanese Patent Application Laid-Open No. 2002-148551 discloses configurations of an optical scanning device for a multi-color image forming apparatus as an example. One of the configurations is such that light beams from light sources for respective colors are collectively scanned by a single polygon mirror, and a plurality of return mirrors are integrally supported in a shared optical housing to guide the light beams to corresponding scanning optical systems and photosensitive drums. Alternatively, the other one of the configurations is such that optical scanning devices are individually arranged in association with the respective photosensitive drums.
In the multi-color image forming apparatus configured in the above manner, components are arranged so that light beams toward the photosensitive drums pass through different paths. Therefore, scanning positions of the light beams easily fluctuate due to environmental factors such as ambient temperature where the multi-color image forming apparatus is placed.
A displacement between scanning positions is periodically detected and corrected using a misregistration detection pattern recorded on a transfer element upon startup of the device or between jobs. However, as explained above, a radiating position further fluctuates due to heat produced by a fixing device and a polygon mirror during printing operation. Thus, if there are a large number of sheets to be printed for one job, color misregistration and color change occur little by little.
Particularly, as described in Japanese Patent Application Laid-Open No. 2002-148551, when the optical systems are arranged to face each other across a polygon mirror, the scanning directions are opposite to each other. Therefore, the write-start position is displaced due to fluctuation of the main-scanning magnification, and besides that, scanning positions between colors are displaced in their increasing direction due to distortion of an optical housing. Accordingly, color misregistration and color change easily occur.
As measures against the problems, when the temperature is always observed and the change in temperature reaches a predetermined value or when the number of prints exceeds a predetermined number, the printing operation is interrupted, and a displacement between the radiating positions is corrected again. However, the processes such that the misregistration detection pattern is created and corrected or that the detection pattern is again created and corrected take several minutes until the operation is finished. Accordingly, productivity is deteriorated. In addition, toner is wastefully consumed because of creation of the detection pattern, and thus, the frequency of correction is desirably suppressed to a minimum.
Recently, to achieve improved scanning characteristics, it has become common to use a specific surface such as an aspheric surface for the imaging optical system of the optical scanning device. Such an imaging optical system is used in many cases because the specific surface can easily be formed and a low-cost resin material can be used to manufacture the imaging optical system.
In the imaging optical system made of the resin material, the optical characteristics are affected and easily changed by changes in temperature and humidity. The changes of the optical characteristics cause the scanning line bending and constant speed performance to be changed. Therefore, when tens of color images are continuously formed, the internal temperature increases due to continuous operation of the image forming apparatus. With this increase, the temperature in an optical writing device increases, and positional precision of a receiving surface of an optical element in the optical housing is degraded. Accordingly, a failure occurs such that a scanning position on the photosensitive element is displaced over time due to changes in the beam position with respect to the optical element and changes in an angle at which the return mirror is provided.
As a result, the optical characteristics of the imaging optical system change, the degree of write scanning line bending of the optical writing device and the constant speed performance gradually change, and color misregistration occurs caused by the displacement between the scanning positions. This leads to completely different color shades between a color image obtained at the beginning of image formation and a color image obtained at the end of the image formation.
A scanning and imaging lens such as an fθ lens, which is a typical scanning optical system, is generally formed as a strip-shaped lens with a long side in the main scanning direction, by cutting off an unnecessary portion (portion into which deflected light beams are not made incident) in the sub-scanning direction.
If the scanning and imaging lens is formed with a plurality of lenses, a long lens is needed. The long lens is such that the lens length in the main scanning direction increases as an arrangement position thereof is farther away from the light deflector. The long lens has a length of about 100 millimeters to 250 millimeters or more. This type of long lens is formed by resin molding using resin material. If temperature distribution in the lens becomes nonuniform due to changes in ambient temperature, a warp occurs, and the lens can be distorted to become a curved shape in the sub-scanning direction. The warp of the long lens in the above manner causes the scanning line bending.
Japanese Patent Application Laid-Open No. 2004-287380 discloses a correcting unit for correcting the scanning line bending due to the warp of the long lens. The correcting unit corrects the bending by pressing the long lens from both sides of the long lens in the sub-scanning direction and adjusting the amount of press force. In the example, a pressing unit is arranged in three locations in the longitudinal direction of the long lens, and a cylindrical pressing element forming the pressing unit is pressed against the long lens, to adjust (correct) the scanning line bending of the long lens.
Japanese Patent Application Laid-Open No. 2005-62258 discloses a unit that selects or moves a pressing position of a curvature adjusting screw provided in the long lens according to the characteristics of the lens, and that presses an inflection point of the curvature to correct the curvature.
In the adjusting (correcting) units, however, the curvature is adjusted by pressing the adjustment position at a pin point. Therefore, the curved position fluctuates due to changes in use environment, or the curved shape is changed to a high-order shape due to the adjustment. Accordingly, the curvature cannot be precisely corrected.
Japanese Patent Application Laid-Open No. 2000-241733 discloses a method of providing anti-vibration by bonding a plate glass to the side face of the long lens along the scanning direction. The method allows improved rigidity of the long lens, and therefore, it is possible to reduce deformation of the lens due to external stress. However, it is difficult to resolve the warp of the lens due to changes in temperature.